How can I identify the ESP used to send a spam email using the email headers?

Key findings

• Header analysis: The most reliable method to identify an ESP is by thoroughly examining the full email headers, looking for specific fields like Received, Return-Path, and X-Mailer or X-Originating-IP.
• IP address lookup: The IP addresses in the Received header can be traced using WHOIS lookups to identify the owner, which is often the ESP. This is a common method for how to find email sending domain details.
• Domain and branding: Some ESPs include their domain in unsubscribe links or view in browser links, providing a direct clue. An article from SendView on finding the ESP highlights this technique.
• Authentication records: SPF and DKIM records in DNS can point to specific ESPs, as they often include their servers in these authentication configurations. Look for domains like include:sendgrid.net or s1._domainkey.mailchimp.com.

Key considerations

• Spoofing attempts: Spammers often spoof legitimate brands, making the From address unreliable. Always refer to the full headers for the true sending origin. Tools like Suped's guide on identifying phishing emails can help.
• Header complexity: Email headers can be long and complex, requiring careful parsing to identify the relevant information. Focusing on the last few Received lines is usually most effective.
• Proxy services: Some spammers use proxy servers or compromised accounts, which can obscure the true origin of the email. This might lead to identifying an intermediate server rather than the initial ESP.
• Dynamic IPs: Consumer ISPs often use dynamic IP addresses, making it harder to track a specific sender. Spam sent from compromised residential IPs is particularly challenging to trace back to an ESP.

Key opinions

• Unsubscribe links: Many marketers point to the unsubscribe or view in browser link domains as a quick way to spot the ESP, as these are often hosted by the sending platform. This is a common heuristic.
• Manual header review: Many marketers advocate for simply viewing the original email and manually checking the headers. This direct approach helps them identify the sending domain and ESP without relying on automated tools.
• Recognizing IP patterns: Experienced marketers often recognize IP ranges or naming conventions associated with specific ESPs from their experience with spam campaigns. This becomes a form of intuition developed over time.
• Tools for quick checks: While manual review is key, marketers also appreciate tools that can parse headers and perform quick IP lookups, streamlining the process of identifying ESPs.

Key considerations

• Time sensitivity: When dealing with persistent spam, marketers prioritize quick identification to facilitate reporting and minimize potential damage. Swift action is often necessary to combat ongoing spam campaigns effectively.
• Brand reputation: If their own brand is being spoofed, marketers are highly motivated to identify the ESP quickly to issue abuse reports and protect their sender reputation. This is crucial for maintaining trust and deliverability.
• Reporting mechanisms: Knowing the ESP allows marketers to direct spam reports to the correct abuse desk, increasing the likelihood of the spammer being shut down. This helps improve the overall email ecosystem.
• Beyond the ESP: While identifying the ESP is a step, marketers understand that the ultimate goal is to stop the spam. This may involve further investigation into the spammers' methods or using email deliverability tests to ensure their legitimate emails are not affected.

Key opinions

• Trust the headers: Experts consistently state that email headers, particularly the Received lines, offer the most accurate path to identifying the ESP. These lines chronologically detail the servers an email passed through.
• Reverse DNS lookup: Performing a reverse DNS lookup on the IP addresses found in the Received header is a common expert technique. This often reveals the domain or hostname associated with the ESP. More information can be found in a SpamResource article on IP reputation.
• Authentication standards: Experts recommend checking SPF, DKIM, and DMARC records related to the sending domain. Misconfigurations or failures in these can indicate a legitimate sender being spoofed or an illegitimate sender trying to bypass filters, and the records themselves often name the ESP. For example, a guide to DMARC, SPF, and DKIM provides context.
• Proprietary headers: Many ESPs add their own X- headers (e.g., X-SG-EID for SendGrid). These unique identifiers can quickly point to the sending platform.

Key considerations

• Evading detection: Spammers constantly evolve their methods to obscure their origins, including using botnets, VPNs, or rapidly changing IP addresses. This makes consistent identification challenging.
• Contextual analysis: Beyond just identifying the ESP, experts also look at the email's content, links, and overall behavior to assess the malicious intent and better understand the spam campaign. This holistic view helps in how to identify spammers.
• Blocklist implications: If a legitimate ESP is being abused, they may find their IPs appearing on blocklists (or blacklists). Identifying the ESP helps in reporting the abuse and potentially getting the ESP to take action. Abusix's blog on ESPs and spam provides further context.
• Persistent monitoring: Effective spam fighting involves continuous monitoring and adaptation, as spammers constantly change their tactics and infrastructure. This proactive approach is key to staying ahead of malicious actors.

Key findings

• The received header: The Received header is added by each server the email passes through. The bottommost Received header (closest to the top of the raw message) indicates the last server before reaching the recipient, while the topmost Received header indicates the first server, typically the ESP's outbound server. This is detailed in Kickbox's guide to email headers.
• Sender authentication fields: Headers include fields like Authentication-Results that show SPF, DKIM, and DMARC verification results. These results can reveal if the sending domain is authorized by its declared ESP or if it's being spoofed. More about this can be found in Microsoft email headers and spam classification.
• IP address traceability: The IP addresses in the Received headers can be mapped to Autonomous System Numbers (ASNs) and their owners via public WHOIS databases. This often directly points to the ESP's network.
• Non-standard headers: ESPs commonly inject proprietary X- headers (e.g., X-Mailer, X-CSA-Complaints) that provide specific ESP-related information or unique campaign IDs. These can be strong identifiers.

Key considerations

• Header forging: While Received headers added by the receiving MTA are generally trustworthy, earlier Received headers and other fields (like From) can be easily forged by spammers. This requires careful scrutiny of the entire header chain.
• Privacy and anonymization: Some legitimate services or spammers might use techniques or services that intentionally obscure the true sender's identity or location, making direct ESP identification challenging. This is part of how spammers use spam traps.
• Mail relaying: Emails can pass through multiple mail servers before reaching the recipient. Each Received line needs to be analyzed in reverse order to identify the original sending server, which is likely part of an ESP's infrastructure.
• Interpreting timestamps: Timestamps in Received headers can help detect inconsistencies or delays that might indicate tampering or unusual routing, further supporting the identification of suspicious emails. Intezer offers a guide to detecting phishing emails using headers.